O'Malley spokeswoman Lis Smith engaged with reporters talking about the mistake on Twitter, saying it shouldn't take away from the substance of the policy paper.

The policy paper and a corresponding open letter are part of a larger push by O'Malley's campaign to define the governor as a tough-on-Wall-Street populist crusader.

"Today, your — too-big-to-fail, too-big-to-manage, and too-big-to-jail — megabanks pose an enormous risk to the financial system, the economy, and American families," O'Malley wrote. "They are so big and so interconnected with the entire financial system that the failure of one or more of them could cause the collapse of the entire US economy."

Democratic presidential frontrunner Hillary Clinton's relationship with top Wall Street institutions has been seen by some liberals as a vulnerability. Though O'Malley has been touting his financial-reform agenda as he has ramped up his presidential campaign, some analysts say that he's been outflanked by Sen. Bernie Sanders (I-Vermont), a longtime outspoken Wall Street critic.